[docs] Document custom static files.

doc/custom_static.md

@@ -0,0 +1,11 @@
+# Custom static files
+Etherpad Lite allows you to include your own static files in the browser, by modifying the files in `static/custom`.
+
+* `index.js` Javascript that'll be run in `/`
+* `index.css` Stylesheet affecting `/`
+* `pad.js` Javascript that'll be run in `/p/:padid`
+* `pad.css` Stylesheet affecting `/p/:padid`
+* `timeslider.js` Javascript that'll be run in `/p/:padid/timeslider`
+* `timeslider.css` Stylesheet affecting `/p/:padid/timeslider`
+* `favicon.ico` Overrides the default favicon.
+* `robots.txt` Overrides the default `robots.txt`.

doc/index.md

@@ -1,4 +1,5 @@
 @include documentation
+@include cusotm_static
 @include api/api
 @include plugins
 @include database